Rising above the diet war between the low fat/high carb and high fat/low carb diet advocates is the aim of a major review of the evidence of whether dietary fat has moved from foe to friend.

The review looked at the cases for both dietary approaches and also the impact of what kind of carbohydrate and what kind of fat people were eating had on obesity and chronic diseases like type 2 diabetes, heart disease and cancer.

It concludes that:

  • Current evidence indicates no specific carbohydrate-to-fat ratio in the diet is best for the general population.
  • Many people can have good health and low chronic disease risk with a broad range of carbohydrate-to-fat ratios.
  • Replacing saturated fats with naturally occurring unsaturated fats (like olive oils and nuts) and replacing highly processed carbohydrates with unprocessed carbohydrates (like whole grains and green vegetables) provides health benefits.
  • All diets, and calorie sources, do not have the same metabolic effects in everyone.
  • For the rapidly rising proportion of the population with severe metabolic dysfunction or diabetes, more specific diets may be needed – and the possible metabolic benefits for such people of ketogenic diets (a specialised low-carb/high fat diet with fat usually making up more than 70% of dietary energy) required long-term studies (See full ‘points of consensus’ below).

The review was prompted by the recent swing against low-fat diets and a resurgence of interest in lower carbohydrate/higher fat and ketogenic diets due to the growing concerns about the adverse effects  on people of diets high in processed carbohydrates.

The review  team of nutrition scientists with “widely varying perspectives” – led by Harvard Medical School and obesity prevention researcher David Ludwig – aimed to summarise existing evidence and find areas of broad consensus amidst the ongoing controversy.

The researchers agreed that no specific fat to carbohydrate ratio is best for everyone, and that an overall high-quality diet that is low in sugar and refined grains will help most people maintain a healthy weight and low chronic disease risk.

The areas they disagreed on, they added, could form the basis of a new nutrition research agenda. These contentious areas included:

  • Do diets with various carbohydrate-to-fat ratios affect body composition (ratio of fat to lean tissue) regardless of caloric intake?
  • Do ketogenic diets provide metabolic benefits beyond those of moderate carbohydrate restriction, and especially for diabetes?
  • What are the optimal amounts of specific types of fat (including saturated fat) in a very-low-carbohydrate diet?

The American scientists expressed concern at the lack of research investment into nutrition research and that the current research agenda had not been adequate to address the important areas of controversy.

“Currently, the United States invests a fraction of a cent on nutrition research for each dollar spent on treatment of diet-related chronic disease, conclude the authors.

“All too often, scientific results in this field have been ambiguous: Macronutrient feeding studies have been too short and too small to distinguish transient from chronic effects; many behavioural trials have lacked the intensity to produce meaningful differences between dietary treatment groups; and observational studies can be affected by confounding, inability to distinguish cause and effect, and other methodological problems.

“Furthermore, despite promising preliminary data, few major studies of a ketogenic diet in the treatment of diabetes have been conducted. Additional questions related to sustainability for the individual (whether people can realistically remain on prescribed diets) and for the environment (the impacts of specific dietary patterns on natural resources and climate change) require more study.

“Given the enormous human and economic toll of diet-related disease, high-quality research into key controversies should be given priority.”

‘Dietary fat: from foe to friend?’ Points of Consensus:

  1. With a focus on nutrient quality, good health and low chronic disease risk can be achieved for many people on diets with a broad range of carbohydrate-to-fat ratios.
  2. Replacement of saturated fat with naturally occurring unsaturated fats provides health benefits for the general population. Industrially produced trans-fats are harmful and should be eliminated. The metabolism of saturated fat may differ on carbohydrate-restricted diets, an issue that requires study.
  3. Replacement of highly processed carbohydrates (including refined grains, potato products, and free sugars) with unprocessed carbohydrates (nonstarchy vegetables, whole fruits, legumes, and whole or minimally processed grains) provides health benefits.
  4. Biological factors appear to influence responses to diets of differing macronutrient composition. People with relatively normal insulin sensitivity and b cell function may do well on diets with a wide range of carbohydrate-to-fat ratios; those with insulin resistance, hypersecretion of insulin, or glucose intolerance may benefit from a lower carbohydrate, higher-fat diet.
  5. A ketogenic diet may confer particular metabolic benefits for some people with abnormal carbohydrate metabolism, a possibility that requires long-term study.
  6. Well-formulated low-carbohydrate, high-fat diets do not require high intakes of protein or animal products. Reduced carbohydrate consumption can be achieved by substituting grains, starchy vegetables, and sugars with nonhydrogenated plant oils, nuts, seeds, avocado, and other high-fat plant foods.
  7. There is broad agreement regarding the fundamental components of a healthful diet that can serve to inform policy, clinical management, and individual dietary choice. Nonetheless, important questions relevant to the epidemics of diet-related chronic disease remain. Greater investment in nutrition research should assume a high priority.

Source: Ludwig et al, Dietary Fat: From foe to friend?, Science 362, 764–770 (2018)

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